1. Field of the Invention
The present invention generally relates to image forming apparatuses, such as copy machines, printers, and multifunction peripherals having multiple image forming functions such as printing and copying functions. Particularly, the present invention relates to an image forming apparatus having a sheet stacking unit.
2. Description of the Related Art
In an image forming apparatus, such as a small-sized printer or copy machine, a dedicated sheet-height sensor may be provided for detecting the height of sheets of recording material (recording medium) stacked in an ejected sheet tray. Based on a signal provided by the sheet-height sensor, the number of sheets ejected onto the sheet stacking unit is controlled in order to prevent stacking failure or the like, as discussed in Japanese Laid-Open Patent Publication No. 2003-137479 (“Patent Document 1”) or Japanese Laid-Open Patent Publication No. 2008-230821 (“Patent Document 2”), for example.
Patent Document 1 discusses a full-status detection filler that includes a filler main body and an auxiliary filler. The filler main body and the auxiliary filler are fastened to each other by screws so that the length of the full-status detection filler can be adjusted depending on the property of the sheet material, such as paper. Patent Document 2 discusses a technology by which plural full-status detection fillers are disposed along a width direction of a sheet in accordance with the size of the sheet.
Japanese Laid-Open Patent Publication No. 2006-256826 (“Patent Document 3”) discusses a technology that attempts to overcome the problem of stacking failure and the like in an image forming apparatus without using a dedicated sheet-height detecting unit. In this technology, a sheet-presence detecting unit provided in the image forming apparatus is used to count the number of sheets ejected onto the sheet stacking unit. The maximum number of sheets that can be stacked in the sheet stacking unit (which may be referred to as a “maximum load”) is varied depending on the type of paper or its basis weight. Specifically, the technology involves setting an upper limit of the number of sheets ejected onto the sheet stacking unit depending on the type of sheet or a print ratio, so that the problem of tacking (where sheets at the bottom of the stack stick to each other) can be prevented.
There is an increasing demand for ever smaller image forming apparatuses. The size of an image forming apparatus may be decreased by decreasing the size of its image forming unit, which may include a process cartridge. However, the size of the image forming unit can be reduced only so much. Thus, there is a trend to focus on how to decrease the size of the sheet stacking unit (ejected sheet tray). However, the decrease in size of the sheet stacking unit naturally results in a decrease in the maximum number of printed sheets that can be stacked in the stacking unit. This means that the risk of stacking failure increases because the maximum load is reached sooner and therefore any excess stacked sheets may fall out of the sheet stacking unit more readily. While such stacking failure may be prevented by providing the sheet height detecting unit as discussed in Patent Document 1 or 2, the sheet height detecting unit leads to an increase in cost and requires additional space for installation.
In the case of Patent Document 3, the existing sheet-presence detecting unit disposed in the sheet transport path is used for counting the number of sheets actually stacked in the stacking unit in order to detect a full-status upon reaching the maximum load without requiring an additional sheet-height detecting unit. However, in the case of small-sized, consumer-oriented image forming apparatuses, the image forming apparatus is usually installed near an operator or a user. Typically, the user goes and picks up the printed sheet immediately after printing partly because such a small-sized printer is not usually used for printing large numbers of sheets at one time.
If the above full-status detection method that counts the number of printed sheets is applied in a small-sized consumer-oriented image forming apparatus, the printing operation may be terminated or interrupted even when the maximum load is not yet reached, thereby adversely affecting the efficiency of the image forming apparatus.
Another problem of the related art is that a printed sheet may be curled in various ways and degrees after fusing, depending on the sheet characteristics and the water content of the sheet. For example, when a side curl develops, the position of the maximum height of the sheet may vary along the sheet width direction, depending on the sheet size. As a result, the maximum height of the sheet may not be accurately detected depending on the location of the sensor or the filler, thus causing stacking failure. While Patent Document 1 proposes adjusting a filler-contacting position depending on the property of the sheet, the shape and degree of a curl may vary over time depending on a moisture-absorbed condition of the sheet in response to a change in ambient humidity. Thus, the shape and degree of curl cannot be determined by the property of the sheet alone. While Patent Document 2 proposes installing the fillers along the sheet width direction to accommodate various sheet sizes, this results in an increase in size and cost of the apparatus and is therefore not suitable for consumer-oriented image forming apparatuses.
The technology discussed in Patent Document 3 attempts to prevent stacking failure without sheet height detection in the sheet stacking unit by utilizing the existing sheet-presence detecting unit to count the number of sheets ejected onto the sheet stacking unit. In this case, the maximum load may be controlled depending on the paper type or the sheet basis weight. While this technology may be capable of preventing stacking failure due to a height detection error, it is not capable of detecting the changes in the shape or degree of curling depending on the moisture-absorbed condition of the sheet, and is therefore not capable of adapting to changes in ambient humidity.